Carburetor CFM Calculator: Find Your Optimal Carburetor Size
Calculate the ideal carburetor CFM (cubic feet per minute) rating for your engine based on displacement, RPM, and volumetric efficiency.
Disclaimer: This calculator provides estimates only. Always consult with a professional mechanic or engine builder before making final carburetor selections for your vehicle. Improper carburetor sizing may affect engine performance and potentially cause damage.
Carburetor CFM Results
Engine Displacement: cubic inches
Maximum RPM:
Volumetric Efficiency: %
Engine Type:
Required CFM:
Understanding Carburetor CFM Calculations
The ToolMasteryHub Carburetor CFM Calculator provides a reliable method to determine the appropriate carburetor size for your engine. The calculator uses the industry-standard formula to compute the required CFM (cubic feet per minute) based on your engine's specifications.
The CFM Formula Explained
For 4-stroke engines, we use:
CFM = (Displacement × RPM × Volumetric Efficiency) ÷ 3456
For 2-stroke engines, we use:
CFM = (Displacement × RPM × Volumetric Efficiency) ÷ 1728
This formula accounts for the three critical factors that determine carburetor size requirements:
- Engine Displacement - The total volume swept by all pistons in cubic inches
- Maximum RPM - The highest engine speed at which full volumetric efficiency is needed
- Volumetric Efficiency - How effectively your engine breathes, expressed as a percentage
Why Proper Carburetor Sizing Matters
Selecting the correct carburetor size is critical for optimal engine performance:
- Too Small - Restricts airflow, causing power loss at higher RPMs and potential engine damage
- Too Large - Results in poor throttle response, rough idling, and inefficient fuel consumption
- Properly Sized - Delivers optimal power throughout the RPM range with good drivability
Interpreting Your Results
Once you've calculated your required CFM, consider these general guidelines:
- Round up to the nearest available carburetor size (common sizes include 390, 600, 650, 750, 850 CFM)
- For street cars, choose a carburetor slightly smaller than calculated for better drivability
- For race applications, match or slightly exceed the calculated CFM for maximum power
- Consider your specific application - drag racing needs differ from daily driving or road racing
Use our free Carburetor CFM Calculator today to help select the ideal carburetor for your engine build or upgrade project!